The present invention relates to the removal of hydrogen sulfide from gas streams using a reaction medium comprising non-aqueous Lewis bases.
Many reservoirs of natural gas contain hydrogen sulfide and carbon dioxide which are acid gases which can be extremely corrosive when combined with each other and water. Natural gas containing such acid or sour gases must be purified (or xe2x80x9csweetenedxe2x80x9d) to remove or decrease the concentration of such gases prior to the purified natural gas (xe2x80x9csweet gasxe2x80x9d) being forwarded to consumer, industrial and other markets.
The most commonly-practised process technology for acid gas removal is the absorption of the acid gases from the natural gas stream by a regenerable absorbing solution in a gas processing plant. In such procedures, a regenerable absorbing solution is passed in countercurrent contact with the natural gas stream to absorb the H2S and CO2, as well as other sulfur compounds, from the natural gas stream, thereby reducing their concentration in the natural gas stream and purifying the natural gas stream.
The acid gas laden solution then is regenerated by steam stripping at elevated temperature and the regenerated solution is cooled and recirculated back to the natural gas contacting stage. Acid gases stripped from the solution in the regeneration step are vented from the gas processing plant for further processing, including, in some case, incineration to sulfur dioxide. The present invention is concerned with a manner of processing sour natural gas streams.
Chemicals that are commonly employed in such procedures include amines, esters and similar regenerable materials in which the acid gases may be absorbed. The most commonly-employed amines for this procedure include monoethanolamine (MEA), diethanolamine (DEA) and methyldiethanolamine (MDEA).
The present invention provides novel procedures for treatment of hydrogen sulfide-containing gas streams. As described in more detail below, the process of the invention includes a step of reacting hydrogen sulfide and sulfur dioxide to form sulfur (sometimes termed the Claus reaction) in a reaction medium comprising a non-aqueous Lewis base, preferably quinoline. The processes described herein are applicable to other gas streams which contain hydrogen sulfide, including Claus process tail gas streams and industrial flue gas streams.
In accordance with the present invention, a reaction medium comprising non-aqueous Lewis bases, having a pKb value of about 6 to about 11, preferably about 8 to about 10, particularly quinoline, are used to remove hydrogen sulfide from gas streams, particularly in the sweetening of sour natural gas streams by effecting reaction between hydrogen sulfide and sulfur dioxide in the reaction medium. The reaction of hydrogen sulfide with sulfur dioxide, which may be in the form of a reaction product with the Lewis base, proceeds in accordance with the equation:
2H2S+SO2xe2x86x923S+2H2O
known as the Claus reaction.
It is well known that sulfur dioxide is soluble in many amines, including quinoline, forming an equi-molar solid reaction product, itself soluble in quinoline and quinoline-water mixtures.
The inventors herein use the reaction product in an original manner to provide improved procedures for removing hydrogen sulfide from gas streams. The present invention uses a reaction comprising a Lewis base which has pKb values from about 6 to about 11, preferably about 8 to about 10. Although strong Lewis bases, (pKb less than about 6) tend to react irreversibly with sulfur dioxide, preventing the Claus reaction from occurring, weaker Lewis bases (pKb greater than about 11) do not appear to catalyze the Claus reaction. The Lewis bases of intermediate basicity (pKb from about 6 to about 11), as used herein, react reversibly with sulfur dioxide and catalyze the Claus reaction. Quinoline (pKb 9) is the preferred amine but other amines with the required pKb values can be used, such as 2,4,6-trimethyl pyridine (pKb 7).
Accordingly, in one aspect of the present invention, there is provided a process for the removal of hydrogen sulfide from a gas stream by reaction with sulfur dioxide, which comprises effecting the reaction in a reaction medium comprising a non-aqueous Lewis base with a pKb value in the range of about 6 to about 11 and which reaction medium:
a) absorbs sulfur dioxide and reacts chemically therewith to form a reaction product;
b) absorbs hydrogen sulfide;
c) removes the hydrogen sulfide from the gas stream through contact of the gas stream with the reaction medium in the presence of free sulfur dioxide, and/or the reaction product;
d) acts as a catalyst for the overall reaction of the hydrogen sulfide with sulfur dioxide to produce sulfur; and
(e) has the capacity to absorb sulfur dioxide in sufficient quantity to remove substantially all the hydrogen sulfide from the gas stream, notwithstanding short term variations in the stoichiometric balance between the hydrogen sulfide and the sulfur dioxide in the reaction medium.
The reaction medium may consist essentially of the non-aqueous Lewis base or may further comprise a miscible diluent of vapour pressure below about 0.39 psi at a temperature of about 120xc2x0 C.
The process of hydrogen sulfide removal provided herein may be effected in a manner in which sulfur dioxide is continuously absorbed by the reaction medium to react with hydrogen sulfide in the gas stream at a temperature of about 120xc2x0 to about 155xc2x0 C., preferably about 120xc2x0 to about 130xc2x0 C., to produce liquid sulfur, and the liquid sulfur so produced is continuously removed from the process.
The latter procedure is particularly useful in a natural gas sweetening operation or for the processing of a hydrogen sulfide-containing gas stream where a continuous operation is required.
The process of hydrogen sulfide removal provided herein may be effected in a manner in which the gas stream is so contacted, intermittently or continuously, with a body of the reaction medium to react the hydrogen sulfide with sulfide dioxide in the reaction product to form sulfur until the reaction medium is depleted of its capacity to react with hydrogen sulfide.
The latter procedure is particularly useful for scavenging operations to remove lesser amounts of hydrogen sulfide on an intermittent operational basis from gas streams having a variety of sources. The procedure may be operated at a temperature above or below the melting point of sulfur and down to the solidification point of the reaction medium. The sulfur usually is permitted to accumulate in the body of the reaction medium until the reaction medium is depleted.
When the reaction medium becomes depleted of the ability to react with hydrogen sulfide, which may be detected by any conventional sensing device, the reaction medium is regenerated. Since regeneration may take a variety of forms, including replacement of the depleted reaction medium by a fresh charge of the reaction medium or a charge of reaction medium regenerated from a previous batch. Regeneration may be effected by reforming the reaction product of the sulfur dioxide and the non-aqueous Lewis base. Sulfur may be removed intermittently as desired from the reaction medium.
In one specific aspect of the present invention, there is provided a continuous process for the removal of hydrogen sulfide from a gas stream, which comprises contacting a reaction medium comprising a non-aqueous Lewis base having a pKb value of about 6 to about 11 having the capacity to absorb sulfur dioxide in sufficient quantity to remove substantially all the hydrogen sulfide from the gas stream, notwithstanding short term variations in the stoichiometric balance between the hydrogen sulfide and the sulfur dioxide in the reaction medium with the gas stream in the presence of sulfur dioxide in the reaction medium to react with the hydrogen sulfide at a temperature above the melting point of sulfur to form liquid sulfur, in accordance with the equation:
xe2x80x832H2S+SO2xe2x86x923S+2H2O
in a upper region of a reactor, accumulating liquid sulfur from the reaction as a layer in a lower region of the reactor below the reaction medium, venting a hydrogen sulfide depleted gas stream from the reactor, and removing liquid sulfur from the layer thereof.
In this procedure, the sulfur dioxide and/or the gas stream may be passed upwardly through the layer of liquid sulfur to remove dissolved components from the liquid sulfur and then through the reaction medium to produce therein the reaction product for reaction with the hydrogen sulfide.
One specific embodiment of the procedure is carried out on a sour natural gas stream containing the hydrogen sulfide. In this specific procedure, the sour natural gas stream first is heated to a temperature at least close to and optionally above the melting point of the sulfur and then is passed to the reactor. The heated sour natural gas stream then is dispersed in the layer of liquid sulfur and is permitted to pass upwardly through the layer of liquid sulfur and into direct contact with the reaction medium containing sulfur dioxide in at least sufficient quantity to convert substantially all the hydrogen sulfide in the gas stream to sulfur.
The resultant sweetened gas stream is removed from the reactor as the vented gas stream. The sweetened gas stream is cooled to remove condensables therefrom and the resulting cooled sweetened gas stream is removed as the product of the process. The heating of the sour gas stream to the temperature may be effected, at least in part, by passing the source in heat exchange relationship with the removed sweetened gas stream, which thereby effects the cooling of the removed sweetened gas stream.
The condensables may be collected and comprise condensed non-aqueous Lewis base, associated compounds and dissolved sulfur and the collected condensables are recycled to the reactor. In addition, the reaction medium may be recycled within the reactor by blending a stream of the reaction medium from the reactor with the collected condensables and recycling the blend to the reactor. The blend may be heated prior to the passage to the reactor. The combined heating of the sour natural gas stream and the heating of the blend may provide the heating required to maintain the reaction temperature in the desired range above the melting point of sulfur.
In another specific aspect of the present invention, there is provided a process for the removal of hydrogen sulfide from a gas stream, which comprises passing the gas stream into a body of regenerable reaction medium comprising a non-aqueous Lewis base having a pKb value of about 6 to about 11 having the capacity to absorb sulfur dioxide in sufficient quantity to remove substantially all the hydrogen sulfide from the gas stream, notwithstanding short term variations in the stoichiometric balance between the hydrogen sulfide and the sulfur dioxide in the reaction medium, and containing a reaction product of sulfur dioxide and the non-aqueous Lewis base to absorb the hydrogen sulfide from the gas stream and to react the absorbed hydrogen sulfide with sulfur dioxide from the reaction product in accordance with the equation:
2H2S+SO2xe2x86x923S+2H2O
to form product sulfur in a reactor, venting a hydrogen sulfide depleted gas stream from an upper portion of the reactor above the reaction medium and permitting the product sulfur to settle to a lower portion of the reactor.
The hydrogen sulfide-containing gas stream may be passed into the body of reaction medium by a gas distributor within the body of reaction medium to distribute the gas stream in the form of small bubbles adjacent to a lower end of the reactor. The procedure may be operated as a continuous process or in intermittent manner and is particularly useful for scavenging operations.
Exhaustion of the capacity of the body of reaction medium to absorb and convert hydrogen sulfide to sulfur may be detected in any convenient manner and the exhausted reaction medium then is replaced with regenerated reaction medium containing the reaction product, or regenerated by the addition of sulfur dioxide. Sulfur may be removed from the reaction medium as required.